Display Sub-Categories (if Exist) on Parent Category otherwise display posts!

Hello, are you there to help me please?
I want my theme to display sub-categories on its one-level up Parent category page (if sub-category exists) otherwise display its child posts.
For ex., suppose there are cat1(as parent), cat2(as sub-cat), cat3(as sub-sub-cat), post1, post2 etc.

Then if some one visits cat1 page then he should get the list of its cat2.
cat1 >> cat2.

if he visits cat2, he should get list of cat3.

cat2 >> cat3.

if finally cat3, he should get the list of its posts.

cat3 >> posts.

Waiting for you kind reply!